KR100325537B1 - Method for manufacturing hot rolled mild steel sheet for press forming - Google Patents

Abstract

PURPOSE: A method for manufacturing a hot rolled mild steel sheet that exhibits superior grain structure homogeneity in width profile by addition of a trace amount of boron and then properly controlling coiling temperature after hot rolling is provided. CONSTITUTION: The method includes the steps of hot rolling a slab comprising C 0.02 to 0.05 wt.%, trace Si, Mn 0.10 to 0.30 wt.%, B 10 to 30 ppm, 0.015 wt.% or less of P, 0.010 wt.% or less S, Al 0.01 to 0.04 wt.% and 40 ppm or less of N to the thickness of 1.3 to 2.3 mm, wherein finish hot rolling temperature is higher than Ar3 transformation temperature; coiling the hot rolled steel sheet at higher than 620 deg.C; pickling the hot coil to remove surface scale; and applying oil on the surface of the pickled steel sheet.

Description

Process for manufacturing hot rolled mild steel sheet for processing

The present invention relates to a method for manufacturing hot rolled mild steel sheet having excellent uniformity in structure. In particular, it has a thickness and mechanical property suitable for press molding while suppressing the formation of uneven mixed structure inevitably generated due to the characteristics of hot rolling process. The present invention relates to a method for manufacturing a thin hot rolled mild steel sheet for processing in the range of -2.3 mm, wherein the hot rolled mild steel sheet or pickling steel sheet of the present invention is usable for the manufacture of drawing parts and elongated molded parts such as refrigerator compressor containers and drums.

Conventionally, the main manufacturing factors for determining the material in the method of manufacturing hot rolled mild steel sheet are known as steel components, hot rolled finishing temperature and winding temperature. The component system used for manufacturing the hot rolled steel sheet for processing is somewhat different from manufacturer to manufacturer, but most of the components are usually 550 to 650 after finishing hot rolling at a temperature of Ar ₃ or higher using a component system of 0.03 to 0.05% C and 0.10 to 30% Mn. It is wound up in the temperature range of ℃. The mechanical properties of the hot rolled steel sheet obtained at this time are generally 45% or more for deep processing and 40% or more for general processing. In addition, even in the same component system, as the thickness of the final product becomes thinner, the material is hardened. For example, the elongation of a 3.5mm thick hot rolled steel sheet manufactured by winding at 600 to 650 ° C using 0.04% -0.15% Mn steel is 45% or more, but at 1.6mm thickness, elongation of 42% is obtained. It is confirmed.

On the other hand, in the case of hot rolled mild steel sheet for processing, in addition to the hardening of the material due to thickness thinning, a problem in application characteristics is the generation of non-uniform structure caused by the widthwise temperature variation peculiar to the hot rolled steel sheet. As the thickness of the hot rolled steel sheet decreases as the rolling temperature decreases, the possibility of finish rolling in the two phases where austenite and ferrite coexist is increased, especially in the strip edge part having a large surface area for the unit volume. In the case of coarse ferrite grains and fine ferrite grains, the possibility of formation of a mixed structure or stretched ferrite grains is much higher than that of the center portion. The elongation of such a mixed tissue is present in the elongation is lower than in the region having a normal tissue and increases the material deviation in the width direction as described above. In addition, in the cold rolling or press forming step of the post-process, it causes a non-uniform deformation, and in severe cases, it acts as a factor of processing crack generation.

Therefore, in order to reduce the mixed structure of the hot rolled mild steel sheet, most hot rolling processes adopt a method of locally heating the edge part by installing a bar edge heater in front of the hot finishing mill. However, even in heating by an edge heater, the temperature which can raise a temperature is limited according to plate shape and rolling thickness. As another method, rolling may be performed by raising the hot rolling finish temperature.

However, in this case, it is known that the possibility of further defects on the surface of the steel sheet increases due to the wear of the rolling roll, the roll roughness following the thermal fatigue, and the increase in the thickness of the oxide scale layer of the steel sheet. On the other hand, the use of both edges of the hot-rolled steel sheet in the end user side by removing the generation of the mixed tissue generation portion of the hot rolled steel sheet has the advantage of minimizing the width-wise structure and material deviation, but there is a problem that the error rate of the steel sheet is lowered than the reduction of the mixed structure It can be seen that a fundamental approach is necessary.

Recently, a method of reducing the mixed structure of a hot rolled mild steel sheet for processing by using a component system for expanding the austenite rolling zone of steel, that is, lowering the Ar ₃ temperature has been proposed. In the case of Japan Keisai, the hot rolled steel sheet for deep processing can be manufactured by adding a large amount of Cr, which is known to be effective in reducing the Ar ₃ transformation point, in order to improve the ductility of steel sheet and prevent material degradation due to the mixed structure. In other words, a hot rolled steel sheet of 1.2-2.3mm was prepared using a component system adjusted to 0.54Cr and 0.001% B, low P, and low Al (0.013%) and low N (22 ppm) in a basic component system of 0.04% -0.1% Mn. As a result of manufacturing, it is reported that it is possible to have the material properties of the tensile strength 33kg / ㎜, elongation of about 48%, in this case there is a problem that the manufacturing cost of the steel sheet is increased due to the addition of a large amount of Cr.

The present invention is to add a very small amount of boron to the low carbon component system in the production of hot rolled mild steel sheet used for drawing and elongation molding, and to control the winding temperature after hot rolling appropriately, the processing thin article having excellent width uniformity in structure in comparison with the conventional hot rolled steel sheet For producing hot rolled steel sheet, yield strength 25kg / mm2 in mechanical properties Tensile Strength 34kg / ㎡ Below, it has a material property of more than 45% elongation and ASTM particle size No. It is an object of the present invention to provide a processing hot-rolled mild steel sheet or pickled steel sheet having a uniform ferrite structure in the range of 8.0-9.0.

1 shows the effect of boron addition on an Ar ₃ transformation point.

2 is a view showing the effect of the winding temperature on the mechanical properties of the inventive steel and comparative steel.

In order to achieve the above object, the present invention provides a method for producing a hot thin rolled steel sheet for processing thin metal having excellent structure uniformity, wherein the weight% is carbon: 0.02 to 0.05%, silicon: trace, manganese: 0.10 to 0.30%, boron: 10 Slabs containing ~ 30ppm, phosphorus <max.0.015%, sulfur <max.0.010, aluminum: 0.01 to 0.04% and nitrogen <40ppm, but in the rolling of hot rolled steel with a thickness of 1.3 to 2.3mm Hot rolling steel sheet for deep processing of 1.3 ~ 2.3mm thickness manufactured by the method of satisfying the above component type and hot rolling conditions as well as controlling the rolling temperature to be above Ar ₃ transformation point and hot rolling after hot rolling. It is achieved by a method for producing a hot thin rolled mild steel sheet for processing, characterized in that oiling is performed after the surface scale is pickled.

Hereinafter, the present invention will be described in more detail.

In the present invention, in the method for manufacturing a hot thin rolled steel sheet or pickling hot rolled mild steel sheet for processing excellent in the widthwise structure uniformity, carbon: 0.02 to 0.05%, silicon: trace, manganese: 0.10 to 0.30%, Rolling was performed on hot rolled steel sheets 1.3 to 2.3 mm thick using slabs containing boron: 10 to 30 ppm, phosphorus <max.0.015%, sulfur <max.0.010, aluminium: 0.01 to 0.04%, and nitrogen <40 ppm In operation, by controlling the filament rolling temperature to be above Ar ₃ transformation point and controlling the coiling temperature to be above 620 ° C after hot rolling, the reduction of Ar ₃ temperature of boron-added steel and the heating of both ends of the bar by the edge heater to generate the composite structure. By further suppressing, the material properties of yield strength 25kg / mm ₂ or less, tensile strength 34kg / mm ₂ or less, elongation of 45% or more and ASTM particle size No. A method for producing a thin hot rolled mild steel sheet for processing having a uniform ferrite grain size in the range of 8.0 to 9.0 is proposed.

In addition, yield strength of 25kg / mm ₂ or less, tensile strength of 34kg /, by pickling after the surface scale of the hot-rolled steel sheet for deep processing of 1.3 ~ 2.3mm thickness manufactured by the method of satisfying the component system and hot rolling conditions. mm ₂ or less, elongation 45% or more, material properties and ASTM particle size No. We propose a method for manufacturing a processing thin pickling hot rolled mild steel sheet having a uniform ferrite grain size in the range of 8.0 to 9.0.

The reason for the numerical limitation will be described below.

The amount of carbon is proposed in order to obtain the appropriate strength and ductility in manufacturing the hot rolled steel sheet for processing, and to comply with the management range of Ar ₃ transformation point, when the carbon amount exceeds 0.05% because the strength of the present invention exceeds the strength range and the elongation is lowered. If it is 0.02% or less, the Ar ₃ transformation point is increased and the possibility of the formation of the mixed structure of the hot rolled steel sheet increases.

The amount of manganese is effective to secure strength with carbon as a solid solution element and to lower the Ar ₃ transformation point, but it is also effective for preventing hybrid structure, but also limited to 0.1 to 0.3% in order to soften the matrix.

Boron is an element which lowers the Ar ₃ transformation point of steel and is very effective in suppressing the formation of the mixed structure. In addition, it is effective in lowering the yield strength because the precipitated boride during hot rolling reduces the amount of solid solution to coarsen ferrite grains. However, when added in excess, the Ar ₃ transformation point decreases in proportion to the added amount, but it was limited to 10 to 30 ppm because it contributes to the increase in the hardenability of austenite and the embrittlement of the steel.

Phosphorus and sulfur are inevitably present as impurities in the steel, but they exceed 0.02% and 0.015%, respectively.

The aluminum is an element necessary for deoxidation of steel, and in general, it is regulated to 0.02 to 0.04% because the amount of aluminum is sufficiently deoxidized if the amount of aluminum is 0.03%.

The amount of nitrogen decreases the ductility similarly to carbon when it is dissolved in steel. In the present invention steel, the nitrogen content is regulated to be within a regulatory range of 40 ppm or less in a low carbon hot rolled steel sheet for processing.

On the other hand, using the high frequency induction bar edge heater in front of the finishing mill is the same as the conventional method. In other words, by compensating the average temperature of the bar edge 25mm by 20 ℃ or more by heating, the mixed structure is formed by making the temperature of the edge of the strip easily above the Ar ₃ transformation point in addition to below the Ar ₃ transformation point according to the composition of the present invention. Rolling was avoided in the austenite and ferrite coexistence zone.

In addition, the coiling temperature was limited to 620 ° C. or higher in order to soften the low carbon hot rolled steel sheet, coarse grain boundary cementite, and to easily precipitate nitrogen in the steel as aluminum nitride (AlN).

It will be described in detail through the following examples.

The chemical composition and the Ar ₃ transformation point of the steel used in the present invention are summarized in Table 1 below. Steel A is for comparing the Ar ₃ change of the steel according to the decrease in the amount of carbon in the steel according to the present invention. It is a low carbon component system. Steels C, D, E, and F are steels in which 8, 11, 20, and 39 ppm are added, respectively, when boron is added. The formation of the mixed structure is very closely related to the Ar ₃ temperature of the steel, and the reduction of the Ar ₃ temperature is effective for suppressing the formation of the mixed structure.

TABLE 1

River Chemical composition Ar ₃ (℃) Remarks C Si Mn P S Al B (ppm) N2 (ppm) A 0.020 0.05 0.14 0.012 0.008 0.036 - 25 883 Comparative Steel, C Lower Limit B 0.038 0.03 0.27 0.011 0.007 0.038 - 32 872 Conventional Steel C 0.040 0.04 0.27 0.010 0.006 0.033 8 36 856 Comparative steel D 0.039 0.05 0.27 0.011 0.006 0.031 11 35 853 Invention steel E 0.040 0.04 0.26 0.012 0.007 0.032 20 33 849 Invention steel F 0.040 0.04 0.25 0.011 0.006 0.032 39 32 840 Comparative Steel, B Upper

A view Figure 1 shows the effect of adding boron to the transformation point Ar _3, Ar ₃ transformation point is lowered in proportion to the added amount of boron in a row shows the 20 ℃ degree degraded by the addition of 20ppm.

After reheating the steel slab containing the component system at 1200 ° C. for 3 hours, hot rolling was performed to produce a hot rolled steel sheet having a thickness of 1.6 mm. At this time, the hot rolling finish target temperature was 870 ° C., and the winding temperature was in the range of 500˜680 ° C. It was. Mechanical properties of the prepared hot-rolled steel sheet was evaluated by pulling at a tensile speed of 10mm / min using a JIS No. 5 tensile test specimen. Secondary brittleness of the steel sheet was evaluated by the formation of brittle cracks when a container with a drawing ratio of 2.16 was applied and an impact load (load 4.44kg, free fall height 1.0mm) was applied at -10 ° C. In addition, the degree of formation of the mixed structure was observed by observing the structure at the edge portion of the final rolled steel sheet when the mixed tissue occupancy was 1/20 or more of the thickness. It is shown by (triangle | delta) and is shown in following Table 2.

TABLE 2

River Winding temperature (℃) Mechanical properties Establishment degree 2nd processing brittleness Legend Remarks Yield strength (kg / mm ² ) Tensile Strength (kg / mm ² ) Elongation (%) ASTM No. B 600 26.4 34.4 42.2 9.9 △ ○ Comparative method 62 25.9 33.4 44.7 9.8 × ○ Conventional Law 680 25.7 31.1 47.2 9.7 × ○ C 500 28.2 34.8 41.4 9.2 △ ○ Yield strength excess elongation lacking Comparative method 620 25.0 32.8 45.2 8.7 △ ○ Mixed occurrence Comparative method 680 24.7 30.7 46.3 8.4 △ ○ Mixed occurrence Comparative method D 500 28.5 34 42.1 8.6 ○ × Yield strength over elongation failure Comparative method 620 23.9 32.2 46.7 8.5 ○ ○ Invention steel 680 23.5 30.2 47.4 8.5 ○ ○ Invention steel E 500 28.5 34.8 41.4 8.6 ○ × Yield strength over elongation failure Comparative method 620 24.1 32.2 44.4 8.6 ○ ○ Invention steel 680 24.0 30.4 45.53 8.5 ○ ○ Invention steel F 500 27.7 33.9 42.4 8.7 ○ × Yield strength over elongation failure Comparative method 620 24.2 31.5 46.7 8.6 ○ × Secondary processing brittleness Comparative method 680 24.6 30.4 47.0 8.7 ○ × Comparative method

In case of steel B of the conventional steel, a material having a tensile strength of 33 kg / mm ₂ and an elongation of about 45% is obtained when the steel B is wound at 620 ° C. or higher, but a mixed structure occurs at the edge of the steel sheet. It started to decrease and was not observed at all above 10 ppm. In addition, when the coiling temperature is higher than 620 ℃ in the extremely small amount of boron-added steel, the ferrite grain size is ASTM particle size No. It is in the range of 8.0 to 9.0, and it can be confirmed that it is larger than steel B of the conventional steel. The change in mechanical properties according to the coiling temperature is shown in FIG. 2. By adding boron in the coiling temperature range of 620 ℃ or higher, the yield strength is reduced by 2kg / mm2, and the tensile strength is about 1kg / mm2, and the elongation is 45% or more. In this section, the decrease in strength, especially yield strength, not only decreases the solid solution nitrogen in the steel as boron is partially precipitated during hot rolling in combination with nitrogen, but also contributes to the precipitation of aluminum nitride (AlN), which contributes to the refinement of ferrite grain size. It is due to coarsening of the ferrite grains because it proceeds earlier. However, as can be seen in FIG. 2, even in boron-added steel, when the coiling temperature is lowered to 500 ° C., the grain boundary cementite becomes fine and its amount is increased so that the yield strength rapidly increases to have a larger value than the conventional steel. On the other hand, as shown in Table 2, the secondary work brittle resistance tends to deteriorate compared to the unadded steel when the amount of boron is increased, which was determined to be due to the refinement of the grain boundary cementite and the increase in number. On the other hand, in the case of steel A of the comparative steel, as shown in Table 1, as the carbon content decreases, the Ar ₃ transformation point is increased, and thus, the degree of hybrid structure generation is similar to that of the steel B of the conventional steel. Considering this result and the result of boron-added steel, it can be seen that the addition of about 20 ppm of boron to the component system of steel A has an effect of reducing the Ar ₃ transformation point by about 10 ° C. compared with steel C of the conventional steel.

Therefore, as described above, in manufacturing the hot-rolled steel sheet for processing, it is weight%, carbon: 0.02 to 0.05%, silicon: trace, manganese: 0.10 to 0.30%, boron: 10 to 30 ppm, phosphorus <max.0.015%, sulfur < max.0.010, aluminium: using a slab containing 0.01 to 0.04% and nitrogen <40 ppm, when rolling on a hot rolled steel sheet with a thickness of 1.3 to 2.3 mm, the filament rolling temperature should be at least Ar ₃ transformation point. By controlling the winding temperature after rolling to be above 620 ℃, the material properties of yield strength 25kg / mm ₂ or less, tensile strength 34kg / mm ₂ or less, elongation 45% or more and ASTM particle size No. It can be seen that the processing hot rolled mild steel sheet having a uniform ferrite structure in the range of 8.0 to 9.0 is produced.

In the manufacture of hot rolled mild steel sheet used for drawing and elongation molding according to the present invention, by adding a very small amount of boron to a low carbon component system and controlling the winding temperature after hot rolling appropriately, it has excellent widthwise uniformity compared to conventional hot rolled steel sheet. The hot rolled mild steel sheet can be manufactured as well as yield strength of 25kg / mm2 in mechanical properties. Tensile Strength 34kg / ㎡ Below, it has a material property of more than 45% elongation and ASTM particle size No. There is an excellent effect of providing a processing hot-rolled mild steel sheet or pickled steel sheet having a uniform ferrite structure in the range of 8.0-9.0.

Claims (2)

In the manufacturing method of the processing thin hot rolled steel sheet excellent in the uniformity of structure, carbon: 0.02 to 0.05%, silicon: trace, manganese: 0.10 to 0.30%, boron: 10 to 30 ppm, phosphorus <max.0.015%, sulfur Using a slab containing <max.0.010, aluminum: 0.01 to 0.04% and nitrogen <40 ppm, in rolling with a hot rolled steel sheet with a thickness of 1.3 to 2.3 mm, the filament rolling temperature should be at least Ar ₃ transformation point and hot rolling. After the winding temperature is controlled to be 620 ℃ or more method for manufacturing a hot thin rolled steel sheet for processing. The method of claim 1, Preparation of a processing thin powder pickling hot rolled steel sheet, characterized in that oiling is performed after pickling the surface scale of the thin-film hot rolled steel sheet for deep processing manufactured by a method satisfying the above components and hot rolling conditions. Way.

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